1. Field of the Invention
The present disclosure relates to a hydraulic control circuit of a transmission, which supplies hydraulic oil to a torque converter of the transmission.
2. Description of the Related Art
An oil pump system including a high-pressure oil passage to which hydraulic oil is supplied from an oil pump at a high pressure, and a low-pressure oil passage to which the hydraulic oil is supplied at a low pressure is commonly known. In this oil pump system, the hydraulic oil is adjusted to a low pressure by a regulator valve when flowed in the low-pressure oil passage, and the oil pressure of the hydraulic oil is adjusted by an oil-pressure adjustment valve when the hydraulic oil is supplied through the high-pressure oil passage into a torque converter having a lock-up mechanism (see Japanese Patent Application Laid-open No. 5-001756).
In a configuration in which hydraulic oil is supplied through a high-pressure oil passage into a torque converter, there is a case where the torque converter does not need to be cooled. In this case, a method of placing a switching valve on the high-pressure oil passage and interrupting supply of the hydraulic oil to the torque converter with the switching valve can be applied. For example, Japanese Patent Application Publication No. 61-25945 describes a configuration of an oil pump system that supplies hydraulic oil to a torque converter, in which a switching valve is placed on an oil passage through which the hydraulic oil is supplied from an oil pump to the torque converter, thereby to reduce drive loss of the oil pump.
However, if the configuration in which the switching valve is placed on the high-pressure oil passage to interrupt supply of the hydraulic oil to the torque converter is added, the following problems occur. That is, if the hydraulic oil is interrupted by the switching valve, loss caused by leak of the hydraulic oil from an oil-pressure adjustment valve placed on the high-pressure oil passage or a selection valve that selectively switches between hydraulic oil in the low-pressure oil passage and hydraulic oil in the high-pressure oil passage, to be supplied to the torque converter may be increased. Furthermore, when the supply of the hydraulic oil to the torque converter is interrupted, supply of the hydraulic oil through the low-pressure oil passage may be also interrupted, which prevents the hydraulic oil from being supplied to constituent members requiring the supply of the hydraulic oil.